Heat screen for projection apparatus



April 7, 1942. E. D. TILLYER HEAT SCREEN FOR PROJECTION APPARATUS Filed May 12, 1939 Patented Alpi'. 7, 1942 HEAT SCREEN FOR PROJECTION APPARATUS Edgar D. Tillyer, Southbridge, Mass., assignor to American Optical Company, Southbridge, YMass., a voluntary association of Massachusetts appucationMay 12, 1939, senat No. 213,363'

(c1. zia-,24)

2 Claims.

This invention relates to improvements in projection apparatus and has particular reference to improved means and methods of forming and supporting a heat screen for the protection of the film or slide used with such apparatus.

One of the principal objects ofthe invention is to provide a substantially colorless vitreous infra-'red absorbing heat screen. having high transmissionof the visible rays and method of supporting the same so as to make it resistant to heat, shock and strain.

Another object of the invention is to provide improved means and methods of forming and supporting a heat screen so as to make it resistant to shock and strain by controlling its composition, mounting it in a resilient support and by heat insulating it from its holder.

Another object of the invention is to provide improved means and methods of forming and supporting a heat screen of vitreous material in such a manner as to make it possible to distribute the absorbed heat evenly throughout the area of said heat screen.

Another object of the inventionis to provide a heat screen which is flexibly supportedin its holder with said flexible support tending to retard the passage of heat from the heat screen to the holder.

Another object of the invention is to provide improved means and methodsof forming and supporting a heat screen of vitreous material which will transmit a high percentage of the visible rays and reduce or eliminate the transmission of the invisible or infra-red heat rays, and which is resiliently supported on and substantially `heat insulated from its holder.

Another object is to provide improvedmeans and methods of forming and supporting a heat' Another object is to provide a heat screen ofV the above character, which comprises a plurality of separate panels, or sections of vitreous material, which are adapted to move under the'action of resilient means upon the application of hea or cold thereto.

Other objects and advantages of the invention should become apparent from the following rangement of parts, and methods shown and described as the preferred forms only have been given by Way of illustration.

Referringto the drawing:

Fig. I Iis a diagrammatic; longitudinal cross sectional view through a projection apparatus,

showing one arrangement and location of the de-l vice embodying the invention;

Fig. II is an enlarged view of the heat screen and support of applicants invention, taken on lines II-II of Fig. I;

Fig. III is a view similar to that of Fig. II, showing an alternate supporting construction;

Fig. IV is a view similar to those of Fig. II and Fig, III in which the heat'screen is made up of separate panels, which are so supported that they may move under the action of resilient means upon the application of heat or'cold; and

Fig. V is a partial section of Fig; IV, taken on line V-V. Y

In the past, many types of heat screen devices have been devised for use with projection machines to obviate the danger of the action of the intense heat created by projectors upon the rel-A A atively combustible means, such as the iilm or slide, or the like used with such projectors. It

was vfound that the best possible material for use in such a heat screen is a vitreous material which balances the desirable elements of greatest light transmission, greatest freedom from color, and greatest heat retardation properties.

Upon using this desirable material, however, it was found that the material was subjected to strain and breakage when heat was applied thereto. Such a disadvantage could easily prove very dangerous and expensive, due to replacement of broken heat screen and damaged film or slide, due to the heat acting thereon'past the broken heat screen. v

The present invention deals with means and methods of supporting the above-mentioned desirable heat screen material in a projector in such a manner as to obviate the dangerous possibilities of strain and breakage., Thus, applicantmakes possible the safe and practical use of the most desirable heat screen material.

Referring more particularly to the drawing wherein like characters of reference designate like parts throughout the several views, the device embodying the invention, asillustrated diagrammatically in Fig. I, comprises a source of illumination I, with'a reflector 2 positioned so as to direct the light from the source of illumination I, through the projector, as is common in the art. The beam of light from the lamp I and the reflector 2 passes through the condenser.

- lenses 3. Its purpose is to allow the visible light to pass `through and act on the film to produce the desired picture, and also to retard the heat generated by such strong illumination as is used in projectors of this sort, so that it will not have the opportunity to harmfully act upon the lm 6. which is of a material which is readily destroyed by excessive heat.

Referring to Fig. II, of which a cross section is shown onthe line II-II in Fig. I, applicants invention, as illustrated for this particular instance, comprises a holder 8, which has an opening 9 therein, and a heat retarding lining material in said opening which is flexible and may be resilient, if so desired. This lining III has a very definite use, in that it tends to keep the heat which is built up in the heat retarding material II bythe action of the beam of light thereon, from leaking off or being carried off by contact between the material II and the holder 8, through the support l0, so as to cause variations in temperature throughout the area of said heat retarding material II, which may be considerable due to the fact that the edge portions of the heat retarding. material II, if in direct contact with the holder 8. would have means of direct conduction of its heat, which the central portions of the heat retarding material II would not have. With the heat retarding lining I0. which is flexible, inserted between the holder 8 and the heat retarding material Il, it will be seen that the heat built up in said heat retarding material II is substantially completely kept therein as far as'conduction through the holder 8 is concerned. This means that the heat built up in the heat retarding material II is carried off by air currents, and by radiation through and on both sides. With this arrangement it will be seen that there will be no portions of the area of said heat retarding material II which are at a substantial variation in temperature with other portions of said material.

This is a very important point, due to the fact that the most desirable heatretarding material for use at II in projectors of this type has been found to be one which is balanced between the material providing the greatest transmission of the visible rays, the one which transmits all the colors substantially without color absorption. and the one which retards the greatest percentage of heat per unit area. Such a material, however, has been found to be subject to strain and breakage if the temperature is at any considerable variance at various portions of its area It will, therefore. be seen that in order to use this desirable heat retarding material, some means must be provided for keeping the temperature of said material constant air, substantially uniformly throughout its area v throughout its area. Applicants invention is directed to this purpose and is broadly accomplished by the insertion between the heat retarding material II and the holder 8, of a heat retarding flexible material I0, such as asbestos. It will be noted that the asbestos lining III lies within the opening 9 of the support 8 against a shulder I2, and that the heat retarding material II is supported by said asbestos liner I0 within or overlying the opening 9, and that a cover plate I3 overlies the heat retarding member II and the asbestos liner I0, and is attached to the holder 3 by means of screws I4, or other suitable attaching means.

The heat retarding material II is preferably formed, in order to have the infra-red or heat absorbing properties and also the heat shock resisting properties which will render said screen or condensing lens resistant to cracking or breaking under the action of heat.of a vitreous composition whose analysis may be expressed as Pros-52 to 77%; material from the group consisting of CaO, BaO, SrO-0.1 to 12%; material from the group consisting of KaO, NazO, Liao- 0.1 to 10%; and material from the group consisting of AlzOa, BeO and mixtures of A1203 and BeC-5 to 20%; Fe3(PO4)z.8H2O-1 to 10%, or by the use of a glass whose chemical analysis may be expressed as:

The material or glass resulting from the above compositions is reduced to a desired size and thickness and to the contour shape desired.

YThis may be accomplished by surfacing the opposite sides of the material with a suitable abrasive such as emery, rouge, etc., for producing the surface texture desired. 'Ihe heat screen may be reduced to the desired size by cutting and edgingv the said glass material by means and methods employed in the art of forming ophthalmic lenses. The screen thus formed may then be hardened to place the outer surfaces in compression and the interior in tension to render the glass material of said screen resistant to heat shock.

'I'his may be accomplished by heating the glass material to a point just about at the softening point and for a predetermined time, depending upon its dimension, and then chilling it rapidly by an air jet, vapor jet, or by dropping the said heated material into a bath of oil, fused salts, or the like.

The amount of heat and time is so controlled as to avoid softening the screen to a point whereby its finished surfaces will flow and'become distorted or the screen change its shape. `'Ihis is of particular importance when one of the condensing lenses 2 is used vas the screen, and is formed of the material having the infra-red absorbing characteristics, as the surface curvareiinished vslightly to insure against distortions or blemishes. This lens will have its outer surfaces in compression and its interior in tension, and will thereby be rendered resistant to heat shock.

It is particularly desirable that the glasses used in forming said heat screens be substantially free from silica and that they contain at least 60% of phosphorus, expressed as P205, and that they contain an infra-red absorbing ingredient, such as iron in the ferrous condition. This produces ay glass which will be relatively clear and transparent and which possesses the desirable infra-red absorbing characteristics.

The screen or lens formed of material whose analysis may be expressed as: P205: material from the group consisting of Ca0, BaO, SrO; material from the group consisting of KzO, NazO, Li20; material from the group consisting of A1203, BeO, and mixtures of A1203 and BeO; and Fe3(PO4)2.8HzO, will be substantially colorless and will have the desired infra-red absorbing characteristics. The lens or screen, therefore, while absorbing the infra-red or heat rays, will havean exceptionally high transmission of the visible rays. Most prior art heat screens of,

this character had from 40% to 50% transmission, while the screen embodying the invention, as formed of the glass described above, will have from 85% to 90% transmission. It is apparent,

therefore, that the change in intensity of the light, as brought about by the heat screen embodying theY invention, will be much less thanv that of the prior art heat screens mentioned above and will be substantially unnoticeable to the human eye.A I

The glass whose chemical analysis is expressed as P205, A1203, F6203, MEO, BaO, B203, S102, and NazO, will have some color, but will transmit much more of the visible rays than most prior art heat screens.

The glasses disclosed herein are resistant to weathering, acids, and to discoloring, particularly when acted upon by radiation.

The gist of the invention, vas herein described, is to provide a colorless heat screen which may be permanently mounted in the path of the projected light, between the light source and the film or slide, and which will have the desirable infra-red or heat absorbing characteristics with a high transmission of the visible rays, and which will be resistant to cracking or breaking when acted upon by heat, due to the fact that it is insulated from its holder by a flexible heat resistant material. such as asbestos. The colorless heat screen set forth above renders it possible to use such haat screens with a projection apparatus, utilizing commercially known Kodatween the source of light and the colored nlm,

and the other end formed to grasp a corner of the heat screen. These braces so support the heat screen in the holder as to leave a space of air between the heat screen Ii and the holder U throughout the periphery of said heat screen.

The braces -I5 may be of wire or other suitable material and is preferably made slightly resilient, so as to take up any shock or strain which might cause the heat screen Il to vibrate or expand or contract. In this alternate structure, it will be noted that there is ample air space between the heat screen and the support, so that direct conduction of heat from the heat screen' to that support would be substantially negligible.

Another alternate construction embodying applicants invention is shown in Fig. IV. In .this construction `the heat screen il is made up of a plurality of separate panels I9 of the desired heat screen material, with the panels arranged parallel to each other and beflng substantially rectangular, as shown. In the recess of the holder 8 is positioned a plurality of springs I6, which bear on the edges of the heat screen panels which are adjacent theed'ges of the opening 9 in the holder 8. It is to be noted that the contacts made by these springs on the heat screen and on thel holder cover a relatively small portion of the edges of said panels and that there by a blow, or expansion, or contraction which acts on the Vheat screen,as shown in this structure, will cause the panels to move under the action of said springs, so as to take up such shock or strain without breakage. This alternate construction is provided with a cover plate as it does not materially/change the intensity or color of said light. It, therefore. will have little, if any, effect on the projected colors as produced by the color ilm slide. This is a big advance over known prior art heat screens of thischaracter, as most prior art screens so reduced the intensity of the light that it was not practical to permanently mount the said screens f in the path of the projected light. Such prior' art screens also greatly altered the color of the light.

I'I which is designed to hold thev heat screen panels `and the springs I6 in position by means of tongues I8. This construction may have cover plates on both sides of the heat screen, as shown in Fig. V. This type of-cover plate allows free circulation of air between'- the heat screen panels I9 and their holder 8, thus providing heat insulation between them.

Both of the alternate structures shown in Figs. III and IV are well heat insulated from their holders,- due to the fact that they are surrounded by air spaces and come in contact with said supports only through the contact members or supporting members which contact a relatively small part of the area of said heat screen, thereby tending to keep the retardedheat of projection from being conducted to the holder 8, and in this manner, keeping the temperature of the heat screen uniform throughout its area, and thus Apreventing strain or breakage, due to sudden variationsin the temperature of different portions of the heat screen.

It is to be understood that the cover plate I3 may be made so. as to contact the heat screen.

- holder and other-parts of the unit only at spaced points of an amount and positional distribution It'is to be understood that the coefiicientfof 75 to properly retain the various-parts oi' the unit expansion of applicants glasses may be conin desired position in order that a suilicient circulation of air may be obtained between the heat screen and holder and throughout the area of said heat screen. A section of such a cover plate is shown in Fig. V.

One of the important features of the invention is the provision of resilient or flexible supporting means for supporting the heat screen member in spaced, heat insulated relation with the holding member. It will be noted that such an arrangement provides means whereby the heat screen member may expand or contract without the danger of strain or breakage which would be present if the heat screen were held in a rigid support.

It is found 'desirable to have the projected beam cover the entire heat screen so that the screen will receive the heat from the beam uniformly over its entire area; thus helping to eliminate temperature differences at different parts of the screen and the resultant strain. This is illustrated in Fig. I Where the path ofthe light beam is outlined by dot and dash lines and the path of that part of the beam which covers the heat screen is outlined by dash lines. It should be understood that the dot and dash lines on the film side of the heat screen (Fig. 1) merely illustrate the path of the light beam if no heat screen were present. This beam is, by the insertion of the heat screen reduced to the path outlined by the dash lines.

From the foregoing description it will be seen that simple, efllcient, and economical means and methods have been provided for accomplishing all of the objects and advantages of the invention, particularly that 'of providing a heat screen of vitreous material having the desirable infrared or heat absorbing characteristics, exceptionally high transmission of the visible rays, being colorless, and being re'siliently supported in and heat insulated from its.holder whereby it will be more resistant to cracking or breaking when acted upon by heat.

Having described my invention, I claim:

l. A heat retarding unit for use with a light projection system comprising a support having an opening therein through which light may be projected, the portion of the support having the opening therein having substantial thickness producing a wall surrounding said opening, said support having a lip surrounding said opening on one side of the wall thereof, a heat screen of a contour shape to ilt within the opening in the support and of a dimension permitting said contour edges thereof to be spaced relative to the wall of the opening throughout-said contour edge, heat retarding means between the edge contour of the heat screen and the wall of the opening having a yielding nature for resiliently engaging and supporting the heat screen and means secured on the side opposite the lip having a portion overlying the adjacent side surface of the heat screen adjacent the contour edge thereof for securing said heat screen and holding said heat retarding means in position and allowing the heat screen to expand or contract while held against displacement against said support.

2. A heat retarding unit for use with a projector embodying means for producing a beam of light and a member for supporting means to be projected having an aperture through which said beam of light is directed, said heat retarding unit comprising a support having an opening therein adapted to be positioned in said beam of light between the means for producing said beam and said member, said opening being larger than the aperture in the member for supportingthe means to be projected, the portion of the support having the opening therein having substantial thickness producing a wall surrounding said opening, a heat screen of a contour shape to t within the opening and of a dimension permitting the contour edges thereof to be spaced relative to the wall of the opening throughout said contour edge, means between the edge contour of the heat screen and the wall of the opening of a material having such characteristics and so associated with the heat screen as to have substantially negligible heat conduction and having a yielding nature for yieldingly engaging the heat screen and means carried by the support for retaining said heat screen in the opening in said support and for holding said means having the yielding nature in position and allowing the heat screen to expand or contract while held against displacement from said support, said heat screen, said beam of light and said aperture inthe member for supporting the. means to be projected being controlled as to related sizes so that said heat screen lies entirely within said beam of light and completely intercepts tlie light passing through said aperture whereby the entire areaoi the screen is subjected to heat of substantially even intensity during use.

EDGAR D. 'I'ILLYER. 

